T h s paper presents the measured performance of a 500 kbitisec. 4-QAM (Quadrature Amplitude Modulation) radio link with 2-branch selection diversity using only one receiver subjected to hardware simulated slow frequency-selective Rayleigh fading. The single coherent receiver input is switched between two uncorrelated Rayleigh-fading signals and the one with higher power is selected. This performance is compared to results obtained from computer simulation.The experimental radio link uses TDM (Time Division Multiplexing) with a coherent receiver. Since the down-link transmitter is on all the time (it may send idle markers or data to other users) the received signal measurements and selection can be performed just before the reception of the user's data burst.The measured performance compares favorably with computer simulation. The effect of delay between measurement/selection and the actual data burst demodulation is also presented. It is found from both experiment and computer simulation that selection diversity is effective in reducing the BER floor caused by frequency-selective fading. As a result, a higher data rate can be supported by a multipath fading channel without employing adaptive equalization. Thus, a low complexity hardware design incorporating selection diversity is feasible for a portable radio communications system.

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T o investigate the viability of TDMA in a fading environment, an experimental radio link has been designed and implemented. It consists of a 500 kb/s differentially encoded 4-level Quadrature Amplitude Modulation (QAM) burst transmitter and a coherent receiver with a fast carrier recovery circuit. The demodulator is a modified Costas camer tracking loop with a dynamic loop (bandwidth switching) filter using hard-decision detection.A bandwidth switching technique, coupled with another technique, i.e. freezing the loop filter between two consecutive bursts so that at the start of a burst only phase must be reacquired, results in very efficient burst coherent detection. A carrieracquisition preamble length equivalent to 7 symbols (28 pec.) has been achieved. Bandwidth switching also helps to recover from a deep fade faster than when a constant narrow bandwidth is used. There is only a small degradation in the radio link performance with increasing user speed (Doppler frequency) in a fading environment.

This paper presents the performance of an experimental TDMA portable radio link with a switching single loop frequency synthesizer. The synthesizer, used as a Local Oscillator (LO), operates around 2 GHz with a 400 KHz step size and can switch over 20 MHz in less than 400 psec. Performance of the TDMA radio link using WER (Word Error Ratio) as a criterion was measured with the synthesizer switching frequencies between bursts at the transmitter. WER versus SNR (Signal-to-Noise Ratio) shows small degradation, only 0.5 dB at WER of due to the phase noise of the synthesizer signal. The phase noise for an offset of 10 KHz and 100 KHz were -70 dBc/Hz and -90 dBc/Hz, respectively.Measurements indicated that the radio link performance showed only minor degradation when the synthesizer is switched 20 MHz only 400 psec before transmitting a data burst. This permits a handset to switch to different channels than the channel of operation to measure signal quality for possible call-transfer once every 2 msec frame.